Broadband arc lamps or flashlamps are often used to excite, or "pump," solid-state laser gain media. These lamps are typically positioned parallel to an elongated crystalline structure doped with a rare earth element such as Neodymium (Nd). Radiation emitted by these lamps is absorbed through the sides of the gain media with the Neodymium atoms absorbing the pump radiation and re-emitting it at the lasing wavelength or wavelengths characteristic of the given gain media.
Surrounding or positioned adjacent the gain media of some relatively simple lasers are two mirrors. These mirrors act generally to form an optical resonant cavity through the gain media, redirecting the re-emitted radiation through the media until lasing results. Depending upon the configuration of the optical cavity, multiple modes of oscillation, or transverse electromagnetic modes (TEMs), may occur. Each TEM has a unique spatial characteristic within the resonant cavity, with the fundamental mode--TEM.sub.00 --both occupying the smallest volume within the cavity and having the lowest divergence. This mode additionally provides optimal focusing properties and displays a uniform Gaussian energy distribution across both the X (nominally horizontal) and Y (nominally vertical) axes. Operation in the TEM.sub.00 mode, accordingly, is desired for virtually all applications of pulsed and continuous wave (CW) lasers.
Broadband arc lamps and flashlamps pump the entire gain media of the laser essentially indiscriminately. As a result, much of the pump energy that these lamps provide is directed to regions of the gain media outside the volume occupied by the TEM.sub.00 mode, not substantially contributing to amplification of the TEM.sub.00 laser beam but instead producing simultaneous oscillation of numerous TEMs and "mixed," or "multi"-mode, output. The efficiency of this pumping technique in converting pump light to the TEM.sub.00 mode output is, therefore, relatively low.
Laser diodes present more efficient pumping sources. Often configured so as to "end" pump the gain media, these diodes can be positioned so that their collective pump volume more closely matches that of the TEM.sub.00 mode. As noted in U.S. Pat. No. 4,908,832 to Baer (which patent is incorporated herein in its entirety by this reference), this end-pumping configuration of the diodes permits the pump direction to coincide with the longitudinal axis of the optical resonant cavity. It also limits the amount of energy that can be used and the resulting power of the laser, however; otherwise, power densities in the pump region of the gain media may become too great and produce heat exceeding that capable of being removed.
Laser Diodes may be used for side pumping gain media as well. As with broadband arc lamps and flashlamps, laser diodes may pump regions of the gain media outside the volume occupied by the TEM.sub.00 mode. In such cases devices such as spatial apertures can be used to eliminate much of the undesirable multi-mode output. Spatial apertures concurrently diminish the power output of the device, however, resulting in decreased performance and efficiency. Suitable diodes for side and end pumping gain media include those specified in the Baer patent and also are available from Laser Diode Products, Inc., 1130 Somerset Street, New Brunswick, N.J. 08901.
Because neither side nor end pumping gain media using laser diodes produces optimal results, additional techniques have been suggested to enhance the power output of these lasing devices. One proposed approach is to pump both ends of the gain media, while another is to utilize multiple gain media within the same cavity configuration. The ultimate power output available using these techniques remains limited by the amount of energy input to the individual pumping sites and the number of accessible sites, however. As the number of sites and gain media increase, these techniques become increasingly difficult to implement effectively.
In the Feb. 1, 1992 issue of Optics Letters, Messrs. Larry R. Marshall, A. Kaz, and R. L. Burnham mention a "three-pass oscillator" for a pulsed laser in an article entitled "Highly efficient TEM.sub.00 operation of transversely diode-pumped Nd:YAG lasers" (the "Fibertek article"). According to the authors of the Fibertek article, the fundamental mode path of the oscillator "traces a Z pattern through the gain volume," allowing "87% of the multimode energy to be extracted in the TEM.sub.00 mode."
As shown schematically in FIG. 3 of the Fibertek article, a single two-dimensional (i.e. stacked) array of laser diodes, at least some of which are connected in parallel, abuts the cylindrical Nd:YAG rod. The small-diameter (3.5 mm) rod thus necessarily collimates the diode radiation itself, limiting the volume of the fundamental mode. The small-diameter rod additionally cannot absorb all of the optical radiation directed into it from the stacked diode array, precluding the gain media from being pumped from more than one side. Although not addressed in the Fibertek article, the rod size further likely cannot support more than three passes through the gain media without degrading the TEM.sub.00 mode output of the device.
As a consequence of these (and other) limitations, the Fibertek article provides only "preliminary" experiments in connection with this technique. These limitations are also consistent with the results reported in the Fibertek article, which recounts higher outputs using a single-pass system having Brewster faces cut on the laser rod than with the Z pattern traced through the gain media. Notwithstanding the disclosure of the Fibertek article, therefore, there continues to exist a need for a scalable, multi-pass pumping mechanism providing greater output energy than conventional end-pumping methods and more efficiency than existing side-pumping techniques.